It is known in the art to use systems of drainage and retention aids comprising high molecular weight (hereafter HMW) charged polymers and anionic inorganic particles, e.g. bentonite and silica based particles. These additives are introduced into the stock in order to facilitate drainage and increase adsorption of fine particles onto the cellulose fibers so that they are retained with the fibers. However, the efficiency of drainage and retention aids usually is deteriorated in stocks which have a high cationic demand due to the presence of interfering anionic substances. In the art, such substances are commonly referred to as anionic trash. The level of anionic trash usually is high in stocks based on recycled fibers and mechanically derived pulps. To counter the deteriorated performance of additives observed in such stocks, it is known to use low molecular weight (hereafter LMW) cationic polymers as anionic trash catchers which are initially added to the stock in order to neutralize the anionic trash and reduce the cationic demand, thereby enhancing the efficiency of drainage and retention aids subsequently added.
European Patent Application No. 0335575 discloses a process for the production of paper which comprises a preliminary polymer inclusion stage in which a LMW cationic polymer is added to a cellulosic suspension followed by addition of a main polymer selected from cationic starch or HMW cationic polymer and then an inorganic material selected from bentonite or colloidal silica.
European Patent Application No. 0308752 relates to a method for making paper which comprises the steps of adding to paper furnish a LMW cationic organic polymer and then colloidal silica and a HMW charged acrylamide copolymer having a molecular weight of at least 500,000.